The present invention relates generally to the field of piezoelectric sensors. In particular, the present invention relates to an improved piezoelectric sensor that is useful to sense pressure variations such as those produced by passing vehicles, i.e., trucks or automobiles or as required by security systems.
Piezoelectric sensors are useful to sense pressure variations. These sensors are useful in a wide variety of applications such as with traffic management features, i.e., the measurement of vehicle speeds, vehicle classification and counting as well as weight in motion. Piezoelectric sensors are used as transducers because a potential difference is generated when the sensor is subject to a pressure change. A detection system is electrically coupled to the piezoelectric sensor and senses, for example, that a vehicle has passed over the sensor.
One known configuration for a piezoelectric sensor includes an inner conductor, an intermediate insulating member formed from a piezoelectric material and an outer conductor surrounding the intermediate insulating member. Certain polymeric materials such as polyvinylidene fluoride (PVDF) have been previously used as a piezoelectric material. However, PVDF has been found to have a relatively low piezoelectric activity unless there is an orientation stage in the manufacturing process. In addition, the PVDF exhibits poor thermal stability. Copolymers of PVDF have been successfully used as a piezoelectric material, but they are substantially more expensive than PVDF. Also, the application of the copolymer can be problematic because the copolymer must be uniformly polarized as it is extruded over the inner conductor.
Therefore, there is a need for an improved piezoelectric sensor that is more reliable than previous constructions. There is also a need for a sensor that is formed from a more economical piezoelectric material.
The present invention is directed to a sensor having an inner conductor, a piezoelectric layer, and an outer conductor. The piezoelectric layer is formed from a piezoelectric strip wrapped around the inner conductor that has adjacent turns. Substantially each adjacent turn overlaps substantially fifty percent of an adjacent turn. The piezoelectric layer has a substantially uniform thickness. The outer conductor substantially surrounds the piezoelectric layer.
According to another aspect of the invention, a sensor is provided having an inner conductor, a piezoelectric layer comprising two piezoelectric strips and an outer conductor. The piezoelectric layer is formed from first and second overlapping portions. The first portion is wrapped around the inner conductor and the second portion is wrapped over the first portion. The piezoelectric layer has a substantially uniform thickness. The outer conductor substantially surrounds the piezoelectric layer.
According to a further aspect of the invention, a sensor is provided having an inner conductor, a piezoelectric layer and an outer conductor. The piezoelectric layer is formed from first and second overlapping portions. The first portion is formed from a first piezoelectric strip wrapped around the inner conductor and having adjacent turns. Substantially each turn of the first piezoelectric strip overlaps substantially fifty percent of an adjacent turn. The second portion is formed from a separate second piezoelectric strip wrapped over the first portion. The second portion of the piezoelectric strip has adjacent turns wherein substantially each turn overlaps substantially fifty percent of an adjacent turn. The piezoelectric layer has a substantially uniform thickness. The outer conductor substantially surrounds the piezoelectric layer.
As used herein, the terms xe2x80x9cfirst portionxe2x80x9d and xe2x80x9csecond portionxe2x80x9d are generally intended to include a piezoelectric material formed from both a single strip of material or two or more strips of material except when the xe2x80x9cfirst portionxe2x80x9d and the xe2x80x9csecond portionxe2x80x9d are stated to be formed from separate elements.
As also used herein, the term xe2x80x9csubstantially fifty percentxe2x80x9d is intended to include an overlap between 40%-60%.
The present invention is directed to a sensor using a piezoelectric layer formed from an economical material that is easily implemented in the sensor. These and other advantages of the invention will be best understood in view of the attached drawings and the following detailed description.